Paper No. 16
Presentation Time: 12:45 PM
WATER-ROCK INTERACTIONS OF ARSENIC IN THE GETTYSBURG BASIN, PENNSYLVANIA, USA
Arsenic is a ubiquitous element throughout the Earth’s crust, but has been found heterogeneously distributed at concentrations above the crustal average of 1.7 mg/kg in crystalline environments such as the Northern Appalachian Mountains (Peters, 2008) and in sedimentary environments such as the Newark and Gettysburg rift basins of New Jersey and Pennsylvania. In addition, groundwaters in both crystalline and sedimentary aquifers have arsenic concentrations above the maximum contaminant level of 10μg/L. The arsenic geochemistry and water-rock interactions of the Northern Appalachian Mountains and the Newark Basin have been researched at length, however little is known about arsenic in the Gettysburg Basin. While the Newark and Gettysburg Basins were both formed during the breakup of Pangea, sediment deposition occurred during the Triassic and lithologies are of similar depositional environment. However, the rocks in the Newark Basin contain a larger percentage of arsenic concentrations above 1.7 mg/kg, ranging from 73% to 95% dependent upon formation, than the Gettysburg Basin. Deposits in the Gettysburg Basin have 18% to 39% of rocks with arsenic above 1.7 mg/kg. Differences are also seen between the basins in the concentrations of arsenic in water with the Newark Basin having overall higher arsenic.
This study takes a detailed look at the rock geochemistry (arsenic, iron, silica and carbon) and water geochemistry (arsenic, nitrate, sulfate, alkalinity and pH) of the Gettysburg Basin to understand its unique properties in the three depositional environments, playa, lacustrine, and bajada and why arsenic concentrations are not as high as the Newark Basin. In addition, rare earth elements (REE) are used to geochemically trace provenance of the basin deposits to understand arsenic source. The water in the playa and lacustrine deposits have arsenic concentrations greater than 10μg/L in 15% and 8% of samples respectively, while 39% of the water samples from the bajada are greater than 10μg/L. The arsenic concentrations in the sampled water in the playa and lacustrine deposits exhibit negative trends with pH, while the bajada deposits have a positive trend, suggesting that arsenic geochemical processes differ between these depositional environments.